This invention relates generally to mattress foundation structures and more particularly to a box spring assembly of a type which utilizes non-coil springs. Box spring assemblies of this general type have been known since 1964, the first such spring assembly being disclosed in U.S. Pat. No. 3,286,281. Subsequently issued patents disclosing the same general type of box spring assembly are: U.S. Pat. Nos. 3,487,480; 3,506,987; 3,574,240; 3,574,241; 3,665,529; 3,680,157; 3,755,833; 3,824,639; and 3,852,838. Box spring assemblies of the general type shown in the above list of patents, all of which are assigned to the assignee of this application, are advantageous with respect to the conventional box spring assemblies using coil springs because they provide a desired stiffer foundation for the mattress and contain a reduced amount of wire. These box spring assemblies are also advantageous from the standpoints of prolonged service life, ease of assembly and cost of manufacture.
Additional box spring assemblies of this general type are shown in U.S. Pat. Nos. 3,596,299; 3,722,013; 3,825,960; 3,833,948; 3,835,485; 3,869,740; 3,990,121; and 4,000,531.
The present invention provides a box spring assembly which utilizes a different spring from the formed wire springs utilized in the patented box spring assemblies discussed above. The spring in the present box spring assembly has an upper portion which yieldably collapses under load to impart a feeling of softness when the bedding occupant first reclines on a mattress supported on the box spring assembly of this invention. This collapse of the upper section is accompanied by a fast build up of resistance to load to reduce the chance of excessive deflection. The upper section then "bottoms out" meaning that it reaches a position in which it cannot yield or collapse any further. All of the bedding load is then carried by the lower section of the spring which consists principally of an upright length of straight spring wire which functions as a column to support the bedding load. The column is substantially unyielding, thus imparting a feeling of firm support to the bedding user following the initial feeling of softness. This combination of initial softness and subsequent firmness is desirable in bedding foundations for reasons of user comfort; the load-carrying capability of the spring being much greater than the initial soft feel would indicate to the user. The lower section also includes a torsion bar which enables some yielding of the lower section to prevent the lower section from taking a set.
It is an object of the present invention, therefore, to provide an improved box spring assembly having limited deflection formed wire springs capable of imparting the desired feeling of comfort to the bedding user.